Compositions for the treatment of magnesium alloys

ABSTRACT

The invention relates to a composition for treating magnesium alloys aimed at improving the resistance thereof to corrosion. The composition is an aqueous solution with a pH ranging between 7 and 10, containing a niobium salt, hydrofluoric acid, and optionally a zirconium salt, phosphoric acid, and boric acid. The alloy is treated in an electrochemical cell in which said alloy acts as an anode. The cell contains an inventive composition at a temperature between 20° C. and 40° C. as an electrolyte. An initial voltage which is sufficient to create a current density between 1.5 and 2.5 A/dm 2  is applied to the cell, whereupon the voltage is progressively increased to a level ranging between 240 and 330 V in order to maintain the initial current density.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to the treatment of magnesium alloys witha view to improving their corrosion resistance.

2. Description of the Related Art:

Magnesium alloys are of great benefit owing to their lightness. Inparticular, they are applicable in the transport (automobile andaeronautical) industries, medical equipment and mobile telephony. One ofthe weak points of these materials is their sensitivity to corrosion.Corrosion protection may be obtained by depositing a coating or bytreating the surface. The protective layers may be produced either bychemical conversion or by anodizing in solutions containing metal saltsand metalloids.

Anodizing treatments have been carried out in electrochemical cells inwhich the electrolyte contains a peroxide or a powerful oxidizing agentsuch as a chromate, a vanadate or a permanganate. One drawback of usingsuch electrolytes lies in the fact that the transition metal ions arepresent, including some that are in the form of toxic species, forexample Cr⁶⁺. Other electrolytes not containing transition metals havebeen proposed. Thus, U.S. Pat. No. 4,978,432 describes the anodizing ofparts made of a magnesium alloy using an electrolyte containing borateand/or sulfate anions introduced in acid form. However, the protectivelayer that forms on the surface of the magnesium alloy part has a highporosity, which impairs the effectiveness of the protection.

SUMMARY OF THE INVENTION

The inventors have found that, surprisingly, the use of an electrolytecontaining a niobium salt in a method for the anodizing treatment of amagnesium alloy part allows an adherent protective layer of very lowporosity to be obtained on the surface of said part.

It is for this reason that the subject of the present invention is acomposition for the anodizing treatment of magnesium alloys, a methodfor treating the magnesium alloys using said composition, and thetreated alloys obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A composition according to the invention for the anodizing treatment ofa magnesium alloy is an aqueous solution containing a niobium salt,hydrofluoric acid, and optionally a zirconium salt, the pH of whichsolution is maintained at a value between 7 and 10, preferably between 8and 9.5.

The niobium salt may be chosen from oxides and fluorides. Niobiumpentoxide is particularly preferred.

It is preferable that the treatment composition be supersaturated withniobium pentoxide, corresponding to a concentration of 0.04 mol/l.Niobium pentoxide forms fluoroniobate (fluoroniobyl) complexes withhydrofluoric acid, these complexes decomposing water with evolution ofhydrogen by being oxidized on the surface cathode sites. The magnesiumions formed on the anode sites react with the niobium complexes orcomplexes of other intermediate species to form, on the surface of themagnesium alloy, mixtures of hydrated niobium oxides (which areinsoluble), of magnesium oxides, optionally of aluminum oxides and ofother species.

Compositions containing a zirconium salt are preferred. The zirconiumsalt may be chosen from oxides and fluorides. ZrF₄ is particularlypreferred.

The pH of the solution is controlled by the addition of compounds suchas NH₄OH or an amine (for example hexamethylenetetramine orhexamethylenediamine).

A treatment composition according to the invention may furthermorecontain other constituents, especially other oxidizing agents such asphosphoric acid and boric acid.

In one particular embodiment, a composition according to the inventionfor the treatment of a magnesium alloy contains:

-   -   from 0.01 to 0.04 mol/l of niobium pentoxide;    -   from 20 to 50 ml/l of hydrofluoric acid;    -   up to 0.04 mol/l of zirconium fluoride;    -   from 50 to 70 g/l of H₃PO₄;    -   from 30 to 70 g/l of H₃BO₄; and    -   the required amount of a 28% aqueous ammonia (NH₃) solution for        adjusting the pH to a value between 7 and 10, preferably between        8.5 and 9.

A composition according to the invention for the treatment of amagnesium alloy part may be obtained by dissolving, with stirring, theniobium salt in a solution containing hydrofluoric acid, then by adding,in succession, the zirconium salt, the phosphoric acid and then theboric acid in the form of an aqueous solution, and then the compoundsintended to adjust the pH, the various steps being carried out withstirring for a time long enough to dissolve the compounds added.

More particularly, a composition according to the invention may beobtained by a multi-step method in which

-   -   the niobium pentoxide is dissolved in a hot aqueous hydrofluoric        acid solution (for example at 50° C.) and with stirring for        about 10 hours;    -   the zirconium salt is added and the mixture is left with        stirring until the salts have dissolved;    -   the phosphoric acid is introduced;    -   the boric acid in the form of an aqueous solution is introduced;        and    -   the pH is adjusted to a value between 8.5 and 9 by the addition        of a 28% aqueous ammonia solution or of an amine such as        hexamethylenetetramine or hexamethylenediamine.

The method of treating a magnesium alloy according to the inventionconsists in making said alloy undergo electrolysis in an electrochemicalcell in which said alloy functions as anode(+), characterized in that:

-   -   the electrochemical cell contains, as electrolyte, a composition        according to the invention at a temperature between 20° C. and        40° C.; and    -   an initial voltage sufficient to create a current density        between 1.5 and 2.5 A/dm², is applied to the cell and then the        voltage is progressively increased up to a value between 240 and        330 V in order to maintain the initial current density.

The duration of the electrolysis is from 5 to 30 minutes, preferablyfrom 15 to 25 minutes.

Preferably, a DC source connected in series to an AC source is used aspower supply for the electrochemical cell so that the I_(AC)/I_(DC)ratio is about 0.15 to 0.30.

It is desirable for the part to be protected to undergo a preliminarysurface cleaning treatment before it is introduced into the electrolysiscell. This preliminary treatment may, for example, consist of amechanical cleaning operation using abrasive disks such as SiC disks,followed by a degreasing operation in a hot phosphate/carbonatesolution, and by a pickling operation in a dilute phosphoricacid/hydrofluoric acid solution or by a degreasing operation and apickling operation.

A magnesium alloy part treated according to the method of the presentinvention has, on its surface, a hard adherent layer containing Zr, Mgand Nb oxides and also Mg and Zr fluorides, phosphates and borates.

The porosity of such a layer is substantially lower than the porosity ofthe layers obtained by the electrolytic treatment methods of the priorart. The low residual porosity may be further reduced by an additionaltreatment, called plugging. The treatment may consist of an alternationof steps in which the part is immersed in a bath and left in air, thesesteps being followed by annealing at 75⁰–150° C. in oxygen for a fewhours. It is advantageous to use, for the plugging bath, an aqueous acidsolution containing niobium pentoxide, a water-soluble cerium salt and azirconium salt, more particularly a composition which has a pH ofbetween 2.4 and 6 and which contains from 0.02 to 0.05 mol/l of niobiumpentoxide, from 1 to 2.5 ml/l of hydrofluoric acid, at most 0.01 mol/lof a zirconium salt and from 0.03 to 0.1 mol/l of a water-soluble ceriumsalt. A preferred composition for the plugging bath is the following:

HF (48% concentration):  2.3 ml/l; Nb₂O₅:  0.04 mol/l; ZrO(NO₃); 2H₂O:0.068 mol/l; Ce(NO₃)₃; 6H₂O: 0.068 mol/l; NH₄F (97% concentration): ~0.3 mol/l.

The plugging may also be carried out in a hot Na₂SiO₃ solution, or withan epoxy/polyamide varnish (for example one sold under the nameFREITAPOX®), or with an epoxy/polyamide paint (for example one soldunder the name VIGOR EP®).

The layers thus obtained may serve as final protection layer or assubstrate for a paint.

The magnesium alloy parts treated according to the method of theinvention have, compared with an untreated part, an improved corrosionresistance. To confirm the improvement in corrosion resistance,specimens were subjected to voltammetry in a corrosive medium (forexample in a 0.5 mol/l Na₂SO₄ solution with polarization). The curvesrepresenting the variation in the current as a function of the potentialin a corrosive medium show a shift in the corrosion potential towardmore positive values and a substantial reduction in the corrosioncurrent and in the anode dissolution current compared with the untreatedalloy.

The present invention will be described in greater detail with the aidof the following examples; however, it is not limited to these examples.

EXAMPLE 1

Preparation of a Treatment Composition

Introduced into about 150 ml of water were 34 ml of 48% HF, then 0.025mol of Nb₂O₅ was added hot, and the mixture stirred for about 10 hoursin order to dissolve the Nb₂O₅. Next, 0.03 mol of ZrF₄ was added hot,with stirring, and the stirring continued for about 24 hours in order tocompletely dissolve the ZrF₄. After the ZrF₄ had been completelydissolved, 60 g of H₃PO₄ were added, followed by 70 g of H₃BO₄ dissolvedbeforehand in water. The pH was adjusted to a value of 8.5–9 by adding a28% aqueous ammonia solution and the amount of water needed to obtainone liter of solution was added.

Treatment of an Alloy

An AZ91D magnesium alloy was treated using the composition obtainedabove. The AZ91D alloy is a magnesium alloy containing 9% aluminum and1% zinc.

The part to be treated was placed in an electrolysis-cell containing theabove composition and said part was connected to the anode(+). Thecathode was made of stainless steel. Next, a potential increasing up toa value between 240 and 330 V was applied between the anode and thecathode in order to maintain the current density at a value between 1.4and 2 A/dm². The current was a DC current on which an AC current wassuperposed. The voltage was maintained for a period of 20 minutes.During anodizing, the treatment solution was stirred and the temperaturewas maintained in the 20–40° C. range by cooling.

Analysis of the Coating Obtained

The layer obtained on the surface of the treated part was dense,homogeneous and of low porosity. XPS analysis showed the presence ofZrO₂, MgF₂, MgO, Nb₂O₅ and NbO_(x)F_(y), Mg phosphates and borates. Thelayer exhibited good adhesion to the alloy substrate.

Analysis of the Performance

The treated alloy part was subjected to measurements by voltammetry andby impedance spectroscopy in a 0.5 mol/l aqueous Na₂SO₄ solution withpolarization. By way of comparison, the same measurements were carriedout on the untreated AZ91D alloy and on the AZ91D alloy treatedaccording to the method of the abovementioned U.S. Pat. No. 4,978,432.The curves obtained show that, in the case of the alloy treatedaccording to the invention, the corrosion current and anode dissolutioncurrent are reduced compared with the untreated alloy and compared withthe alloy treated according to the prior art. These results wereconfirmed by the impedance spectroscopy.

EXAMPLE 2

Preparation of a Treatment Composition

34 ml of 48% HF were introduced into about 150 ml of water, and heated,then 0.035 mol of Nb₂Os was added and the mixture was subjected tomagnetic stirring for about 10 hours in order to dissolve the Nb₂O₅.Next, 58 g of H₃PO₄ were added followed by 70 g of H₃BO₄ dissolvedbeforehand in water. The pH was adjusted to a value of 8.5–9 using a 28%aqueous ammonia solution and the total volume of the solution was madeup to 1 liter by adding the appropriate amount of water.

Treatment of an Alloy

An AZ91D magnesium alloy identical to that used in Example 1 was treatedunder the conditions described in Example 1 using the above compositionand then the treated parts were annealed at 150° C.

Analysis of the Coating Obtained

The layer obtained on the surface of the treated part is dense,homogeneous and of low porosity. XPS analysis shows the presence ofMgF₂, MgO, Nb₂O₅ and NbO_(x)F_(y), phosphates and borates. The layerexhibits good adhesion to the alloy substrate.

Analysis of the Performance

The treated alloy part was subjected to measurements by voltammetry andby impedance spectroscopy in a 0.5 mol/l aqueous Na₂SO₄ solution withpolarization.

By way of comparison, the same measurements were carried out on the parttreated according to Example 2 and then subjected to annealing at 150°C., on the same alloy treated according to the method of U.S. Pat. No.4,978,432 and subjected to annealing. The curves show that, for thealloy treated according to the invention, the corrosion currents arereduced compared with the alloy treated according to the method of U.S.Pat. No. 4,978,432. These results were confirmed by the impedancespectroscopy.

1. A composition for an anodizing treatment of a magnesium alloy,wherein the composition comprises an aqueous solution, containing aniobium salt and hydrofluoric acid, the pH of which solution ismaintained at a value between 7 and 10, and wherein the niobium salt isan oxide or fluoride.
 2. The composition as claimed in claim 1, whereinthe niobium salt is niobium pentoxide.
 3. The composition as claimed inclaim 2, wherein the composition is supersaturated with niobiumpentoxide.
 4. The composition as claimed in claim 1, wherein thecomposition further comprises a zirconium salt.
 5. The composition asclaimed in claim 4, wherein the zirconium salt is an oxide or fluoride.6. The composition as claimed in claim 4, wherein the zirconium salt isZrF₄.
 7. The composition as claimed in claim 1, wherein the pH isbetween 8 and 9.5.
 8. The composition as claimed in claim 1, wherein thecomposition further comprises phosphoric acid and/or boric acid.
 9. Thecomposition as claimed in claim 1, wherein the composition furthercomprises NH₄OH or an amine for maintaining the pH.
 10. The compositionas claimed in claim 1, wherein the composition comprises: from 0.01 to0.04 mol/l of niobium pentoxide; from 20 to 50 ml/l of hydrofluoricacid; up to 0.04 mol/l of zirconium fluoride; from 50 to 70 g/l ofH3PO₄; from 30 to 70 g/l of H3BO₄; and the required amount of a 28%aqueous ammonia solution for adjusting the pH to a value between 7 and10.
 11. A method of treating a magnesium alloy comprising making saidalloy undergo electrolysis in an electrochemical cell in which saidalloy functions as anode(+), wherein: the electrochemical cell contains,as electrolyte, a composition according to claim 10 at a temperaturebetween 20° C. and 40° C.; and an initial voltage sufficient to create acurrent density between 1.5 and 2.5 A/dm², is applied to the cell andthen the voltage is progressively increased up to a value between 240and 330 V in order to maintain the initial current density.
 12. A methodof treating a magnesium alloy comprising making said alloy undergoelectrolysis in an electrochemical cell in which said alloy functions asanode(+), wherein: the electrochemical cell contains, as electrolyte, acomposition according to claim 1 at a temperature between 20° C. and 40°C.; and an initial voltage sufficient to create a current densitybetween 1.5 and 2.5 A/dm², is applied to the cell and then the voltageis progressively increased up to a value between 240 and 330 V in orderto maintain the initial current density.
 13. The method as claimed inclaim 12, wherein a DC source connected in series to an AC source isused as power supply for the electrochemical cell so that theI_(AC)/I_(DC) ratio is about 0.15 to 0.30.
 14. The method as claimed inclaim 12, wherein the duration of the electrolysis is from 5 to 30minutes.
 15. The method as claimed in claim 12, wherein, during apreliminary step, the alloy part to be treated is subjected to a surfacecleaning operation.
 16. The method as claimed in claim 15, wherein thesurface cleaning operation is a mechanical cleaning operation usingabrasive disks, followed by a degreasing operation in a hotphosphate/carbonate solution, and by a pickling operation in a dilutephosphoric acid/hydrofluoric acid solution; or a degreasing operationfollowed by a pickling operation.
 17. The method as claimed in claim 12,wherein the electrolysis is followed by a plugging treatment.
 18. Themethod as claimed in claim 17, wherein the plugging treatment consistsof an alternation of steps in which the alloy part is immersed in a bathand then left in air, these steps being followed by annealing at75°–150° C. in oxygen for a few hours.
 19. The method as claimed inclaim 18, wherein the plugging treatment is carried out using an aqueousacid solution containing niobium pentoxide, cerium nitrate and zirconylnitrate; a hot aqueous Na₂SiO₃ solution; or an epoxy/polyamide varnishor an epoxy/amine paint.